This project is concerned with (a) analyses of the genetic diversity of medically important parasitic protozoa and its implications to the epidemiology, course and diagnosis of disease, (b) the development of high resolution flow cytometry and quantitative low-light-level video microscopy instrumentation for biomedical research and (c) the utilization of flow cytometry and low-light-level video microscopy for the analyses of infectious agents. We have found that DNA variants of Trypanosoma cruzi appear within a cloned stock at low frequency. The DNA of these variants is unstable. Variant populations contain members with high proportions of non-nuclear, non-kinetoplast DNA in the form of particles in the cytoplasm. Environmental variables such as temperature can act to destabilize the DNA of T. cruzi resulting in the appearance of variant stocks. These observations imply that cytokinesis without karyokinesis followed by DNA stabilization is occurring in this species. The ability of T. cruzi to form viable hybrids in this manner could promote intra- specific heterogeneity and provide a basis for the diversity observed in the presentation and course of Chagas' disease. Low-light-level video microscopy studies are elucidating the chemical composition of the parasitophorous vacuole formed during the infection of erythrocytes by malaria merozoites.